1. Field of the Invention
This invention relates to guided electromagnetic wave transmission systems and, more particularly, to phase changing or phase shifting devices for use in such systems.
2. Description of the Prior Art
It is well known that a round or square waveguide section that produces a 90-degree differential phase shift between two orthogonal waves (quarter-wave plate) may be used to convert an electromagnetic wave either from linear to circular polarization or from circular to linear polarization. It is also well known that a 180-degree differential phase shift section (half-wave plate) may be used to change the sense of a circularly polarized electromagnetic wave, e.g., from right circularly polarized to left circularly polarized.
In U.S. Pat No. 2,438,119 issued to Fox, a system is described which makes use of these properties in order to provide an adjustable phase changer which changes the phase of an electromagnetic wave while producing no change in the polarization of the wave. Such a system uses a rotatable 180-degree phase shift section interposed between two 90-degree phase shift sections. A similar phase changer, described by Fox in U.S. Pat. No. 2,787,765, has been implemented using a constant transverse magnetic field to excite an element of ferromagnetic material in order to effect an electrically controlled or adjustable phase change with no change in polarization.
It is often desired to be able to change the phase of electromagnetic waves travelling or propagating in one direction in a controlled manner while providing no variable phase change for such waves travelling or propagating in the opposite direction, i.e., to have a unidirectional phase changer.
For example, it is often desired to have an antenna pattern that scans for one directon of signal flow only as where a changing phase shift is desired in the receive direction of the antenna for scanning purposes and where constant phase is desired in the transmit direction for providing an on axis beam.
There are many other types of microwave signal processing applications in which a device providing such a signal flow, i.e., unidirectional variable phase, is useful.
In the prior art, such a signal flow is provided, for example, in a device including a combination of junction circulators and phase shifters. Typically, however, circulators provide poor signal isolation, which results in leakage of the input signal, causing some residual phase change in the non-phase shifting direction, and limiting the accuracy of the phase shifting direction.